dxf: add tidb_max_dist_task_nodes to specify max node count

[tangenta]

What problem does this PR solve?

Issue Number: close #58944

Problem Summary: See #58944

What changed and how does it work?

• Add system variable tidb_max_dist_task_nodes
• Add new column max_node_count to mysql.tidb_global_task
• Add simple tests for balance subtasks.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  

  mysql> admin show ddl jobs;
  +--------+---------+--------------------------+--------------+--------------+-----------+----------+-----------+----------------------------+----------------------------+----------------------------+--------+-------------------------------+
  | JOB_ID | DB_NAME | TABLE_NAME               | JOB_TYPE     | SCHEMA_STATE | SCHEMA_ID | TABLE_ID | ROW_COUNT | CREATE_TIME                | START_TIME                 | END_TIME                   | STATE  | COMMENTS                      |
  +--------+---------+--------------------------+--------------+--------------+-----------+----------+-----------+----------------------------+----------------------------+----------------------------+--------+-------------------------------+
  |    118 | test    | sbtest1                  | add index    | public       |         2 |      112 |  10000000 | 2025-02-10 03:09:51.852000 | 2025-02-10 03:09:51.852000 | 2025-02-10 03:10:06.053000 | synced | ingest, DXF, max_node_count=5 |
  |    117 | test    | sbtest1                  | drop index   | none         |         2 |      112 |         0 | 2025-02-10 03:09:48.003000 | 2025-02-10 03:09:48.052000 | 2025-02-10 03:09:48.052000 | synced |                               |
  |    116 | test    | sbtest1                  | add index    | public       |         2 |      112 |  10000000 | 2025-02-10 03:07:56.502000 | 2025-02-10 03:07:56.552000 | 2025-02-10 03:08:10.752000 | synced | ingest, DXF, max_node_count=3 |
  

• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

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None

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[codecov]

Codecov Report

Attention: Patch coverage is 71.21212% with 19 lines in your changes missing coverage. Please review.

Project coverage is 74.7723%. Comparing base (d0d4876) to head (6762c1c).
Report is 53 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #58937        +/-   ##
================================================
+ Coverage   73.0457%   74.7723%   +1.7266%     
================================================
  Files          1689       1736        +47     
  Lines        467009     480976     +13967     
================================================
+ Hits         341130     359637     +18507     
+ Misses       104894      98766      -6128     
- Partials      20985      22573      +1588     

Flag	Coverage Δ
integration	48.7782% <35.3846%> (?)
unit	72.2062% <71.2121%> (-0.0271%)	⬇️

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Components	Coverage Δ
dumpling	52.6910% <ø> (ø)
parser	∅ <ø> (∅)
br	60.3559% <ø> (+14.9248%)	⬆️

[CbcWestwolf]

Rest LGTM

[CbcWestwolf]

Should we specify the default value for this column?

[tangenta]

The default value should be zero or a large integer so that it does not affect the behavior of an existing task.

[D3Hunter]

we can specify the default value be 0, so there is no need to check nil for tasks before upgrade

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-01-22 12:17:02.805148029 +0000 UTC m=+269550.136067433: ☑️ agreed by CbcWestwolf.

[D3Hunter]

we can specify the default value be 0, so there is no need to check nil for tasks before upgrade

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: CbcWestwolf
Once this PR has been reviewed and has the lgtm label, please assign bornchanger, gmhdbjd, yudongusa for approval, ensuring that each of them provides their approval before proceeding. For more information see the Code Review Process.

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Needs approval from an approver in each of these files:

• OWNERS [CbcWestwolf]
• br/OWNERS
• pkg/ddl/OWNERS
• pkg/disttask/OWNERS
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• pkg/sessionctx/variable/OWNERS

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[D3Hunter]

29/35

[D3Hunter]

after change this might incorrectly takes some node as dead:

• we have 4 nodes, in order of N1/N2.... task T have 2 subtasks, and they are scheduled to N3/N4 initially, and are running
• when N1/N2 are available, this PR will choose to use them to schedule subtasks of T, and in L163(have the log dead node or not have enough slots, schedule subtasks away) of doBalanceSubtasks N3/N4 will be taken as dead node, and those running subtasks will be scheduled away